Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory, among others.
Flash memory devices are utilized as non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption.
Uses for flash memory include memory for personal computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code and system data, such as a basic input/output system (BIOS), are typically stored in flash memory devices. This information can be used in personal computer systems, among others.
Two common types of flash memory array architectures are the “NAND” and “NOR” architectures, so called for the logical form in which the basic memory cell configuration of each is arranged
A NAND array architecture arranges its array of floating gate memory cells in a matrix such that the gates of each floating gate memory cell of the array are coupled by rows to word select lines. However each memory cell is not directly coupled to a column bit line by its drain. Instead, the memory cells of the array are coupled together in series, source to drain, between a source line and a column bit line.
Memory cells in a NAND array architecture can be configured, e.g., programmed, to a desired state. That is, electric charge can be placed on or removed from the floating gate of a memory cell to put the cell into a number of stored states. For example, a single level cell (SLC) can represent two binary states, e.g., 1 or 0. Flash memory cells can also store more than two binary states, e.g., 1111, 0111, 0011, 1011, 1001, 0001, 0101, 1101, 1100, 0100, 0000, 1000, 1010, 0010, 0110, and 1110. Such cells may be referred to as multi state memory cells, multibit cells, or multilevel cells (MLCs). MLCs can allow the manufacture of higher density memories without increasing the number of memory cells since each cell can represent more than one bit. MLCs can have more than one programmed state, e.g., a cell capable of representing four bits can have fifteen programmed states and an erased state.
The state of a memory cell, e.g., the data stored in the cell, is determined by the threshold voltage (Vt) of the cell. A MLC includes multiple Vt distribution ranges within a programming window. The programming window represents the voltage range within which the Vts of the memory cells are electrically altered to represent the different logical states. As an example, a programming window can have a lowermost voltage of about −3V and an uppermost voltage of about 3V.
In MLCs, the memory density depends on the number Vt distributions within the programming window, but the Vt distributions must be sufficiently spaced apart so as to reduce the possibility of a higher voltage of one distribution overlapping a lower Vt of the next distribution. For a given programming window, an increase in the number of Vt distributions, e.g., program states, leads to an increase in the number of bits a cell can represent.
One method to utilize the full range of a given programming window, e.g., from −3V to 3V, can include applying negative voltages to word lines of selected cells in order to read and/or verify the Vt levels of the cells programmed to negative Vt levels. However, supplying negative voltages via word line drivers can add circuit complexity and can increase power consumption, among various other problems.